Automatic labeling is of interest to the produce industry, in which it has become a common practice to label each item of produce with some item-specific information, printed in the form of, for example, text or a bar code. The information about the produce may include, for example, its type, size, date harvested, geographic origin, and whether or not the produce is organic. In particular, it has become desirable to label each item with a Price Look Up (“PLU”) number, which enables retailers to facilitate quick handling and accurate pricing of produce at checkout. However, in the past, labeling items with different PLU numbers, for example, denoting “small,” “medium,” or “large” size designations for apples, has required three separate labeling machines, three separate label designs, and three label inventories. Consequently, it has become desirable to be able to apply variable, programmable, information “on the fly” to a produce label tailored to an individual item, thereby requiring only a single labeling machine and only a single, at least partially blank, label design. More background regarding this approach can be found at col. 1 line 11 through col. 2 line 45 of Hirst et al., U.S. Pat. No. 7,168,472, entitled Method and Apparatus for Applying Variable Coded Labels to Items of Produce, which issued Jan. 30, 2007 (hereinafter “Hirst”), the entire disclosure of which is hereby incorporated by reference herein, and at paragraphs 2-21 of Griffen et al., U.S. Patent Application Publication No. 2007/0068630, entitled Multi-Layer Markable Media and Method and Apparatus for Using Same, which was published Mar. 29, 2007 (hereinafter “Griffen”), the entire disclosure of which is also hereby incorporated by reference herein.
As disclosed in both Hirst and Griffen, it is desirable to write variable information directly onto a label using a light beam. To do this in a rapid, consistent, and cost effective manner presents challenges arising from the relationships between the labeling machine, label material, and light beam optics. In particular, it is desirable to provide a high power light beam so as to reduce the required label exposure time. It is also desirable to provide a light beam that has a long depth of focus at the label so as to ensure that a focused image will be written on the label despite potentially significant variations in the label position, relative to the nominal image surface of the light beam optics. It is further desirable to minimize aberrations in the light beam to provide, as nearly as practical, a diffraction limited light beam image at that image surface.
One method and apparatus for direct writing of a pattern with a laser beam is described in Tamkin, U.S. Pat. No. 6,084,706 (hereinafter “Tamkin”). Tamkin discloses a three-mirror afocal optical system in which the mirrors may have aspheric (e.g., parabolic, hyperbolic, or elliptical) or spherical surfaces. Such an all-reflective architecture, which uses mirrors instead of lenses throughout, achieves a high level of transmission efficiency compared to a lens-based system, in which the lens medium inevitably absorbs significant light energy at certain wavelengths.
In general, an afocal optical system is an optical system in which both the object and the image are assumed to be located at infinity. Light rays entering and leaving an afocal optical system are parallel. Examples include binoculars and telescopes, in which the image, although magnified by the optical system, is focused by the eye. Magnification may increase or decrease (i.e., fractionally magnify) the size of the image, depending on whether a magnification factor is greater than or less than one, respectively. An afocal optical system may be formed by combining two focal optical systems so that the rear focal point of the first system coincides with the front focal point of the second system, yielding an overall system that has no effective focal length. Several embodiments of a three-mirror afocal system are described in Tamkin, each having different magnifications.
In Tamkin, a single laser source and a beam splitter are used to produce up to eight separate beams, which are then passed through an optical system to produce a 15,000-pixel image, having pixel sizes in the range of about 1-10 microns. The three-mirror afocal system is then used to relay the scan beams with a desired magnification and minimal loss of power. However, splitting the power of a single laser into multiple scan beams greatly reduces the power that can be delivered per unit time to a given spot on an object, such as a label, thereby affecting the throughput of a direct scan system. In addition, Tamkin does not address the challenges of achieving the long depth of focus required in an automatic “on-the-fly” labeling system.
A multiple laser diode array may be used in a direct write application, rather than splitting a single laser into multiple beams, as disclosed in Landsman, U.S. Pat. No. 6,640,713. However, unless the laser diode array can be placed immediately adjacent the light markable medium, as is the case in writing produce labels on the fly, effective delivery of the laser light to the medium remains a challenge.
Johnson, U.S. Pat. No. 6,177,980 (hereinafter “Johnson”), discloses an optical system that couples an array of miniature lens elements, or lenslets, with an image projection system in a low resolution, large field microlithography application. Johnson modulates the expanded beam of a single diode laser source using a grating light valve or an array of micromirrors. The modulated light is then focused by an array of lenslets into widely spaced point images. The beam separation between the lenslets in Johnson is substantially wider than the focused spot, which requires a writing strategy that is not suitable for high-speed, in-line, web-fed processes. While Johnson discloses the use of an afocal system with an array of lenslets in a direct writing application, it does not address the aforementioned challenges that exist in the design of a direct write imaging system in which the position of the image plane may change significantly with time, the initial quality of the beam is poor, as in the output of a multi-mode diode laser, the illumination power of the beam must be high, and a physically compact, cost effective optical package is desirable.
Accordingly, there is a need for an improved optical system for photosensitive printing by direct writing with a laser beam on a light markable medium, wherein the position of that medium may vary significantly, the illumination power is high, and the optical system should be compact and cost effective.